Not being acquainted with the work of author H.P. Lovecraft, the original inspiration of the Cthulhu Mythos, I found a website to explain the stories.
The following extract is from The Official Cthulhu Mythos FAQ - Part 1
"The Cthulhu Mythos comprises a 'shared world' in which various authors set works of horror, and there is some overlap with the fields of fantasy, science fiction, and mainstream fiction thrown in. ...the Cthulhu Mythos is not controlled by some central authority which commissions books, maintains continuity, and so forth.
Rather, authors use elements from each others' tales in ways which complement (and sometimes conflict) with other's stories. All of these authors, in one way or another, are paying homage to H. P. Lovecraft."
More information at Alt.Horror.Cthulhu FAQ
The setting for Cyaegha's story
The creature is confined and restrained underground, beneath a village called Freihausgarten in a remote part of western Germany. The villagers are a cult that worships Cyaegha and holds bloody rituals of human sacrifice. Cyaegha isn't impressed by this, and - like all self-respecting evil monsters - impatiently awaits a chance to be free to wreak havoc over the land *shriek!*
Did you notice that in the close up photo of those tats, there is - amongst the black tentacles on his left arm - the reddish pink shape of a worm? I don't think it is that now but it was a nice idea.
Here's part of a publicity flyer Marilyn illustrated for the last Spooky Kids show on 31 July 1992. The lettering has a distinct 'tentacle' shape, which made me think he produced his own drawing of Cyaegha for the tattoo artist.
And that's what happened. Donovan explains: 'He brought me the art, and I made stencils from them. The second one ... I think I freehanded', (his left arm). Trivia: each tat was $50 plus a six pack of Heineken.
Albert Sgambati made some additions to these tattoos. In the credits on MM's first album, Portrait Of An American Family, there is the line, 'Tattoos: Albert Sgambati, Donovan'.